fix: call onpropertyChanged with empy device label during mda events

[dario-bassi]

Hi,
i was working with a virtual-microscope, and encountered an error message using an MDA event. After every frame, it was trying to call _on_property_changed with an empty device name after the shutter device was closed. If the device has no label, probably it should skip the call on this signal, since no device state was updated.

[tlambert03]

makes sense! thanks

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.43%. Comparing base (034b7eb) to head (667f297).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #556   +/-   ##
=======================================
  Coverage   87.43%   87.43%           
=======================================
  Files         115      115           
  Lines       14091    14093    +2     
=======================================
+ Hits        12320    12322    +2     
  Misses       1771     1771           

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